.....daggerboards and centerboards
Any object floating on the water
will tend to be blown across the water in a leeward direction, or away from the
direction which the wind is blowing. To prevent this from happening to a
sailboat, and to enable it to sail more in a direction into the wind, a
DAGGERBOARD or CENTERBOARD is used. A daggerboard or centerboard is a thin
vertical plate sticking out the bottom of the boat that gives the boat a
"grip" on the water thereby preventing or at least minimizing leeward
movement. Contrary to popular belief, the daggerboard or centerboard does not
keep the sailboat from tipping over. While daggerboards and centerboards are
not strictly a part of the rigging in terms of its "transmission"
function, they do help greatly in converting the power of the wind in the sails
to forward motion. And because daggerboards and centerboards often require
fittings and lines to make them operable, they are relevant to our discussion
on rigging.
What's the difference between a
daggerboard and a centerboard? Well, a daggerboard can be likened to an actual
dagger that fits into its sheath or case. The "case" in the instance
of the daggerboard is referred to as the DAGGERBOARD TRUNK. The daggerboard
slips in and out of the daggerboard trunk, usually in a vertical direction
only. The purpose of the daggerboard trunk is to provide bearing for the board
and keep water out of the boat. Once the daggerboard is in the trunk there is
very little movement of the daggerboard. The daggerboard can be made out of
sheet metal (usually aluminum) or wood. The hole in the bottom of the boat
where the daggerboard passes through is usually a fairly tight fit around the
daggerboard. There is very little adjustment with a daggerboard. Once it is
down in position, the position can only be varied up or down.
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FIG. 1O-1 - A typical centerboard trunk in a small
sailboat. Part of the centerboard can be seen protruding out the top of the
trunk. The centerboard pennant passes through a sheave on the forward trunk
upright and is knotted to the centerboard. This arrangement forms a
"tackle" to ease the effort required to lift the board. A cleat on
the centerboard trunk is used to secure the pennant. |
A centerboard on the other hand is
hinged at its forward portion on a CENTERBOARD PIN. The centerboard pin is
usually a bolt or rod about which the centerboard pivots up or down in the
CENTERBOARD TRUNK (see Fig. 10-1). The centerboard trunk is similar in function
to the daggerboard trunk. However, with a centerboard, it remains in the boat
at all times in its trunk, while the daggerboard is removable through the top
of the daggerboard trunk. Whereas the daggerboard moves up and down vertically
in the slot of the daggerboard trunk, the centerboard "swings" about
the centerboard pin. Because it can swing, the position of the centerboard can
be varied to suit different courses of sailing, and changes in the loading and
balance of the hull. From this standpoint, a centerboard is more efficient than
a daggerboard. However, the slot for the centerboard where it passes through
the bottom of the boat must necessarily be longer than for a daggerboard, and
this adds to the resistance of the hull thereby slowing the boat down somewhat.
One drawback of daggerboards is that should the boat sail into shallow waters
or accidentally come up onto a beach, the daggerboard will not pivot back and
up into the trunk as will a centerboard. The damage to the boat in such an
encounter could be extensive, whereas with a centerboard, it would pivot back
and up into the trunk with probably no damage at all.
When daggerboards or centerboards
are made from wood, they have a tendency to float up out of the water just like
a wood rudder. There are several ways to prevent this. Naturally, the board
could be made from metal (aluminum or steel) or fiberglass, but this is not
recommended unless the boat is originally equipped this way, or the designer
specifies it. With daggerboards, spring stainless steel "clips" are
available, which fit onto the board making the fit so tight in the trunk that
the board cannot float up. Another method is to use an elastic "shock
cord" across the top of the trunk slot secured at each end to screw eyes
each side of the trunk. With both types of board, a removable "pin"
can be used through the trunk sides and board that will "lock" the
board in position. The boat shown in Fig. 6-8 features such a pin for the
daggerboard trunk. By providing a series of holes in the board, the pin
arrangement could be used to lock the board in various positions to suit
sailing conditions. On centerboards of wood, a common method is to put a weight
on the board. This can be a lead weight built right into a hole in the board,
or a specially fabricated casting designed to fit the bottom of the board. The
weight will pull the board down in position and then a pin arrangement can be
used to lock the board in position.
Daggerboards are simpler in nature
than centerboards, and therefore require few if any fittings other than those
described above. Centerboards, however, can get more complex, especially the
bigger the board gets in size and weight. The line used to raise and lower the
centerboard is known as the CENTERBOARD PENNANT. On lightweight boards, the
line may be attached by a knot through a hole in the board, and lead to a cleat
somewhere on the trunk or hull structure. On heavier boards, the centerboard
pennant can be rigged in a tackle arrangement to decrease the effort required
to raise the centerboard. Pulleys or sheaves can be fastened to the centerboard
and/or to the trunk to come up with the proper tackle.
On many of the trailerable cruising
sailboats from about 17' in length and up, the centerboard is often a piece
of steel plate or even a casting of steel or lead. Sometimes the latter are
referred to as "retractable keels" because they also add to the
stability of the boat. In either case, the weight of the unit is often such
that a tackle will not be practical. When this is the case, a winch is used
with a wire rope pennant to raise and lower the centerboard. The winch is best
a two-way type with positive ratchet action on both the "down" and
"up" directions. Worm-gear winches are available for use on boat
trailers that are suitable for centerboard use, as are more deluxe types
especially designed for centerboard use. All components used in such an
installation should be of extra heavy duty and arranged so no jamming or
fouling of the gear will result in use.
As noted previously, the centerboard
or daggerboard trunk helps keep water out of the boat. This means that no holes
or leaks should be permitted in the trunks AT LEAST BELOW THE WATERLINE.
Sometimes this is a problem with centerboard trunks, especially at the
centerboard pin. The centerboard is constantly "working" and causing
stress on the pin that tends to loosen and let water seep into the boat. There
are so many variations in the design of centerboard pin mechanisms (probably in
an attempt to make a good one that won't leak) that it is not possible to
specify a "cure-all" which will work in all cases. If the pin area is
leaking, check to make sure that the bolt is tight, and if it has worked to
where it fits too loosely in the hole, it should be replaced with a larger pin.
Also check the gaskets, if so equipped, to see that they are in good condition,
and replace if necessary. The structure of the centerboard trunk should be
checked to see that all members are tight and well fastened. Sometimes a
leaking pin is the result of a weakened trunk structure. If this is the case,
repairs or replacement of structural members may be in order. Sometimes a
little fiberglass tape and resin can work wonders in patching up leaks and
reinforcing weak members, but make sure the areas to be fiberglassed are clean
and dry first. Because the inside of the trunks will not be accessible after
assembly, it is advisable to fiberglass the inside prior to assembly to seal
the trunk and protect against abrasion by the board.
In use of the boat, sometimes there
will be a vibration or "chatter," especially noticeable when the boat
is moving along at top speed. This chatter is usually caused by an unfair
centerboard or daggerboard, and sometimes an unfair rudder. What this means is
that the edges and general sectional shape through the board have not been
finished correctly. Obviously a board with a square edge forward or aft will
not go through the water as effectively as a board with edges that are shaped
or "faired." With sheet metal centerboards or daggerboards, about the
only fairing that can be done is to radius the edges. If wood is used, a better
"shape" can be incorporated. One method of fairing a wood board that
gives good all around results is shown in Fig. 10-2. While very fine edges give
least resistance, it is best not to make them "razor sharp" because
they damage too easily in use. It is possible to build up the correct
"shape" to any board by using foam shaped to suit and covered with
fiberglass. If fiberglassing the outside of the hull, it is best to tuck
fiberglass cloth into the trunk to protect the exposed plywood edge.
FIG. 10-2 - A section through a "faired"
wood daggerboard. The same principle can be applied to rudders and
centerboards. Note that the thickest part is located approximately 1/3 the
width of the board aft of the forward or leading edge. Edges should be radiused
slightly to prevent damage.
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With daggerboards that tend to bind
or jam in use, a little paraffin will usually smooth things out. Sometimes the
board will warp or the slot will not be quite the same shape as the board. This
may require reshaping or replacing the board. Sometimes the board will fit too
loosely in the slot. This will cause working at the pin on centerboards, and
sometimes banging from side to side. Small blocks or metal angles can be used
to make the fit at the slot tighter. Some device is often provided, especially
with steel centerboards, to limit the forward travel of the centerboard to keep
it from banging against the forward portion of the centerboard trunk. This is a
good idea because pennants sometimes break, or a lock pin will fail, and the
board will fall forward at a crashing speed.
Chapter 11
Contents
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